The report shows that STDs are on the rise across the U.S. Southern states are particularly affected, although sharp increases in Western states also raise eyebrows.

Overall, 19 million Americans catch an STD every year. Half of these infections are in people aged 15 to 24. At particular risk are teenage girls and young women, African-Americans, and men who have sex with men.

"STDs represent a substantial threat to Americans," John M. Douglas Jr., MD, director of the CDC's Division of Sexually Transmitted Disease Prevention, said at a news conference held to announce the findings.

The CDC report focused on three very troubling STDs: chlamydia, gonorrhea, and syphilis. All of these infections can be cured -- if they're detected. But most people carrying and spreading these infections don't even know they have them.

In the case of HIV, that could be fatal. But all of these diseases can cause permanent damage if left untreated. For example, untreated chlamydia -- the most common reportable infection in the U.S. -- can rob a woman of her fertility.

"Very few young, sexually active women are actually aware that annual chlamydia screening is recommended, and that the disease is linked to compromises in fertility," Stuart Berman, MD, chief of epidemiology and surveillance for the CDC's STD branch, said at the news conference.

America's No. 1 STD: Chlamydia

The CDC received reports of more than 1 million chlamydia infections in 2006, up from 2005. CDC experts believe that the actual infection rate is closer to 3 million.

The increase in the chlamydia rate may represent more actual infections, but CDC researchers aren't sure. Douglas says most of the increase in reported infections is probably due to increased screening for the disease.

What makes chlamydia particularly frustrating is that many women who are treated for the infection quickly get reinfected by their male partners.

"Up to 25% of women treated for chlamydia get reinfected within three to six months," Douglas says. "There is a sense among doctors that once they report the disease to the local health department, they will follow up with the partners. This is absolutely not true for chlamydia and gonorrhea."

The situation is so bad the CDC has started urging doctors to give young women a dose of antibiotics to take home to their sex partners. They're calling this "expedited partner therapy."

"With expedited partner therapy, which is the partner receiving antibiotic treatment without a formal medical evaluation, we have shown we can reduce rates of reinfection if women deliver the therapy to their partners," Douglas said. "We have completed a legal analysis, and now there are 11 states where we have formally determined it is legal. In other states, laws are not in support or are ambiguous. This is a relatively important prevention approach."

But the biggest problem with chlamydia is people who don't think they're at risk.

"The finding that chlamydia rates are highest in young women is almost universal," Berman said. "If health care providers think the young women in their practice don't have chlamydia, they should think again."

The Specter of Drug-Resistant Gonorrhea

Doctors only gradually became aware of the significance of chlamydia in the 1980s. They've known about gonorrhea far longer. And, since a relatively quick way to test for the infection became available in the 1970s, public health officials have been fighting the disease.

That battle looked successful in the years 1975-1997, which saw a 75% drop in gonorrhea infections. But now rates are going up again. One big problem is the rise of drug-resistant gonorrhea.

Nearly 14% of gonorrhea infections in 2006 were resistant to the fluoroquinolone antibiotics traditionally used to fight the disease. That means the drugs should no longer be used against gonorrhea, Douglas says.

That's a big problem, because there's only one remaining class of antibiotics left to fight gonorrhea: cephalosporins. Fortunately, Douglas says, researchers have not yet detected a strain of gonorrhea resistant to this drug.

But he says other germs have learned to resist the drug and that there's no reason gonorrhea could not eventually learn this trick.

"When that happens, it can be a sudden and abrupt change from drug-susceptible to not susceptible at all," Douglas says. "What would happen if we lost this class of drugs? We would have to turn to classes of antibiotics not previously explored against gonorrhea. That might have challenges such as requiring multiple doses or multiple drug combinations."

Syphilis Making a Comeback

No STD frustrates public health officials more than syphilis. It's long been the most dreaded of STDs, because if left untreated the disease can cause an astonishing number of terrible outcomes -- including blindness, mental derangement, and death.

The CDC has been at war with syphilis since the 1940s. For a while, victory was in sight. Syphilis rates hit a historic low in 2000. Since then, they've been on the rise. From 2005 to 2006, syphilis rates jumped 13.8%.

"The syphilis numbers are real and concerning, not in terms of massive population impact but because this is a disease that had been knocked off its feet -- near elimination -- and we have seen reverses in what could have been a preventable problem," Douglas says. "Trends in syphilis clearly are going in the wrong direction."

Driving the syphilis epidemic is a 64% increase in cases among men who have sex with men. But there's also been a troubling increase in syphilis among women. That's led to the first increase in 14 years in the number of babies who contract syphilis in the womb.

Another disturbing factor is that syphilis rates are still much higher among African-Americans than among whites. The situation is better than it was in 1999, when the rate was 29 times higher in African-Americans than among white Americans -- but a sixfold disparity remains.